GALLIUM
G. H. WAGNER and W. H. GITZEN
Aluminum Company of America East St. Louis, Illinois
Oellium cry.t.1.
ALTHOUGH gallium was discovered over 75 years ago
PROPERTIES
(1875), it is only within the past few years that the Gallium, atomic number 31, has an atomic l=ight of metal has advanced beyond the stage of a laboratory curiosity and has become available commercially. ~h~ 69.72. I t belongs in the boron-aluminum family along of this have challenged inves- with indium and thallium. Consequently, it shows tigators in many laboratories in this country abroad marked resemblance to aluminum in most of its chemical but its commercial utilization still lies in the future. properties but differs in several important respects. Gallium is generally stated to be soluble in sodium HISTORY hydroxide and in hydrochloric, nitric, and sulfuric The element gallium was discovered and isolated in acids. With high-purity metal, however, the rate of France in 1875 by Boisbaudran, a brilliant investigator attack by these reagents decreases markedly with inin the field of spectroscopy (1). He noted as early as creasing purity of t,he gallium beyond 99.9 per cent. 1863 that the spectral lines of the boron-aluminum Aqua regia is one of the best solvents for the metal. family form patterns of the same type, with regular Gallium salts are generally prepared from the pure variations from one element to the next. However, metal, since it is usually more advantageous to purify there was a gap between indium and aluminum which the metal than the salts. The solution of the metal in Boisbaudran correctly interpreted to indicate an undis- a mixture of nitric and hydrochloric acids can be neucovered element. He immediately began to search tralized with ammonium hydroxide or ammonium carfor it and was rewarded with success twelve years later. bonate to yield a precipitate suitable for preparing I n 1875, while examining with the spectroscope pome many gallium salts by solution in the corresponding samples of zinc blende from the Pyrenees, Boisbaudrau acids. If anhydrous gallium chloride is desired, it is noticed two intense lines in the violet part of the spark best prepmed by heating the metal in a stream of spectrum. These wave lengths, of about 4172 and 4033 anhydrous hydrochloric acid or chlorine, and recovering A,, were ascribed to element. H~ successfully the gallium chloride as a sublimate. Anhydrous gallium isolated hydrated gallium oxide from the zinc ore by chloride is very hygrosco~ic. chemical methods and obtained the metal by electrolHot, concentrated perchloric acid dissolves gallium ysis of a solution of this compound in caustic potash. rapidly and, on cooling, hydrated gallium perchlorate Gallium is of particular interest because it was the precipitates nearly quantitatively (2). This is said to first element to be discovered by the aid of its spark be satisfactory starting material for the preparation of spectrum and was also the first to he discovered of the other gallium compounds. The usual precautions three "eka" elements predicted by Mendeleev five years should be observed to prevent contact of the perchloric previously. The close agreement of the properties acid, or the crystals, with organic matter, because of of the newly discovered element with the predicted the danger of explosion. The filtrate from the crystals properties for eka-aluminum encouraged general accept- is more concentrated than the perchloric acid used ance of Mendeleev's periodic system. (72 per cent), on account of the removal of water from 162
APRIL. 1952 Phvsical Pronerties Atomic number Atomic weight Crystal structure Melting point, "C. Boiling point, "C. Specific gravity, g./ml. Solid (29.6"C.) Liquid 32.38T.
301 600
31 69.72 Orthorhombio
29.i5 1983 5.904 6.093 5.905 5.720 5 fin5
1805 Sperific heat, cal./g. ('C.) Solid (0" t o 24°C.) Liquid (12.5"t o 200°C.) Latcnt heat of fusion, cal./g. Surface tension, dynes/cm. Volumc resistivity (microhms-cm.) Sohd (0°C.) Liquid (30.3"C.)
400. 0.09 0 082 19 16 735 53 2i.2
the solution by the crystallization of the gallium perchlorate hexahydrate. Gallium usually has a valence of three but compounds with lower valence can be formed. Trivalent hydmxides, oxides, and salts resemble the corresponding compounds of aluminum. Gallium also forms a rather extensive series of metallo-organic compounds. The most common hydrate of gallium oxide is the monohydrate, Ga2O3HzO-isomorphous with d i a s p o r e which is converted to the anhydrous oxide on calcination. Laubengayer and Engle (9) reported the preparation of gallia trihydrate but Foster and Stumpf (4) were unable to obtain the trihydrate and concluded that only the monohydrate exists. Like aluminum, gallium is amphoteric. However, gallium hydroxide has stronger acid properties and weaker basic properties than aluminum hydroxide (5). Also, like aluminum, gallium forms alums. Metallic gallium, hourever, shows little resemblance to aluminum in physical properties. It melts a t 29.75 "C. Only two metals have lower melting points-mercury, melting at -39'C. and cesium, at 28.5"C. I t is remarkable that, notwithstanding the low melting point of gallium, its boiling point is high. Values reportedvaryfrom 1983' (6) to 2070°C. (7), derived from comparatively low-temperature vapor-pressure determinations. The density of gallium increases from 5.904 (solid) at 29.6% to 6.095 g./ml. (liquid) a t 29.8'C. Additional physical properties are given in the table. Gallium, unlike most metals but like water, expands on freezing (3.2 per cent). Other metals that expand on freezing are bismuth and germanium. Since gallium may freeze and melt a t temperatures frequently met in transit, packaging for shipment or
storage obviously requires containers capable of expanding. Gelatin capsules have been used to package small amounts, up to 50 g., and rubber bulbs or polyethylene bottles for larger quantities. Fortunately, polyethylene is one of the few materials not readily wet by liquid gallium, hence losses resulting from wetting are minimized. When pure, gallium is a silvery-white metal with a slight bluish color, and the liquid metal, like mercury, has a brilliant mirror surface. Gallium alloys readily with many metals at temperatures between its melting point and about GOO°C. Its rapid alloying action with aluminum may be demonstrated by draming a streak with a crystal of gallium across a thin aluminum sheet. Intergranular peuetration causes the aluminum to disintegrate within a few minutes, alloxing the sheet to be pulled apart easily along the streak. Tungsten resists attack by gallium below 800°C. (6). Alloys with indium, tin, zinc, cadmium, and aluminum have low melting temperatures, in some cases lower than that of gallium. The copper-gallium and silver-gallium alloy systems have been investigated extensively. Small amounts of gallium have a marked influence on the properties of several metals. The electrical resistivity of copper is raised from 1.677 microhm-cm. to 2.830 microhm-cm. at 18°C. by the addition of 0.845 atom per cent. Up to 5 per cent gallium improves the strength and ductility of magnesium alloys. The resistance of magnesium to corrosion is also improved (8, 9). Gallium inhibits the development of coarse grain during the heat treatment of Mg-Al-Zn-Be alloys. Liquid gallium is soluble in mercury only to about 1.3 per cent at 35% (10). High-purity liquid gallium has a marked tendency to supercool and frequently can be held for days in an ice bath without crystallizing. However, if a small crystal of gallium, or even ice, is introduced, crystallization will take place rapidly. In this manner several hundred grams of gallium in an evaporating dish can be
Gallium Meltins at Body Tempcratvre
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converted into a mass of beautiful orthorhombic crystals in a few minutes (see the figure).
It has been suggested that the concentration of gallium in the British coal deposits may have been the result of biological accumulators of the element. Steinberg claimed that it is essential to growth of the fungus, Aspergillus niger (15), closely related to the mold from which penicillin is obtained, as well as Lemna minor, a small marsh plant (16). RECOVERY
Gellium Crystallizing from Ice-Cooled Liquid Metal
Single, large, orthorhomhic crystals of gallium can he obtained easily and may be grown as rods by crystallizing in tubes. Crystals of gallium &ow marked differences in electrical resistivity and thermal expansion along the direction of the three crystallographic axes (11). Electrical resistance varies as 1:3.2:7 and coefficient of thermal expansion as 31:16:11. The variation in electrical resistivity is believed to he greater than for any other metal. OCCURRENCE
In the Bayer process for recovery of alumina, the gallium dissolves in caustic soda solution along with the alumina and accumulates in the cycling solutions of the Bayer process to an equilibrium point. It will thus he seen that the sodium aluminate solution in every Bayer plant constitutes a source of gallium from which it can be recovered, but a t high cost, because of the small amounts present and the amount of processing necessary. Although the Bayer liquor contains only a very low concentrationof gallium,it is perhaps the best potensource of s u.. u ~-l.vsince , the sodium aluminate solution in the Bayer plants on this continent totals many millions of gallons. Most of the dissolved alumina may be separated by precipitation from the supersaturated Bayer solution. The gallia can then be co-precipitated with the remaining alumina by neutralization with carbon dioxide t o obtain a gallia-rich concentrate (17). This concentrate is dissolved in caustic soda and electrolyzed at temperatures near the boiling point. The gallium is deposited at the cathode as liquid metal, collects in a pool a t the bottom of the cell, and is withdrawn periodically. The crude metal so produced contains small amounts of several impurities-zinc, copper, lead, iron, etc.-and can be ~urifiedhv reoeated electrolvsis from sodium hydroxide solution, by chemical washing with acids, or by recrystallization. The chemical washing method is satisfactory for removing most of the impurities from gallium. It consists of alternate treatments with concentrated nitric and hydrochloric acids, washing the first acid out completely, before treating with the second (18). Nitric acid disperses gallium into very fine droplets, whereas hydrochloric acid causes coalescence. Hoffman (19) at the Bureau of Standards reported a purity of 99.999 per cent for gallium purified by fractional crystallization as the final step. Two general methods for recovery of gallium from zinc ores may he used: (1) Where a chloridizing roast is used for the zinc sulfide concentrates, most of the gallium is volatilized and finds its way to the cadmium recovery plant where it is recovered, usually along with germanium. (2) If a chloridizing roast is not used, the gallium remains behind in the retort residue, from which it may be recovered by re-solution and electrolysis. A method for recovery of gallium from flue dust from producer systems of gas works has been described by Reynolds (18). It consists of fusion of the ash in caustic soda, leaching of the fusion, and precipitation of a gallium concentrate, which may be redissolved in caustic soda and electrolyzed. "
Gallium, like aluminum, is a widely distributed element, but whereas aluminum occurs in tremendous tonnages in many deposits, gallium is seldom found in more than trace amounts, usually below 0.01 per cent. Gallium occurs in many common ores, in meteorites, in sea and mineral waters, in petrified mood and some vegetation. No minerals containing substantial amounts of gallium have been found. The richest known mineral is germanite, which is said to contain up to 1per cent gallium together with 5 to 8 per cent germanium. Although gallium usually occurs only in trace amounts, Goldschmidt (13) estimates that the abundance of gallium in the earth's crust is about the same as that of lead and molybdenum, 15 g. per ton, with aluminum occupying first place as the most abundant metallic element at 88,000 g. per ton. One of the most common occurrences is in bauxite, in amounts rangingfrom about 0.002 to 0.01 per cent. Another common occurrence is in zinc ores, which have been used for a number of years for production of gallium (IS). A promising source of gallium is the flue dust from certain coals, particularly those of Britain. I t is stated that dusts from producer systems of gasworks in Britain contain as much as 1.5 per cent of both gallium and germanium (14).
A
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While gallium can be recovered by electrolysis of an acid solution, the use of sodium hydroxide solution appears preferable and is generally used for production of sallium. Foreign investigators have shown considerable interest in sallium durincr recent vears. I n Russia an intensive survey of mineral sources was undertaken (20), and methods for recovery of gallium from the waste of aluminum production have been reported (21).
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ANALYSIS
Gallium, along with ferric iron and a number of other metals, can be separated from many elements by extraction from a 6 N hydrochloric acid solution with ethyl ether (22). Molybdenum can be separated previous to the ether extraction byprecipitation with &enzoin-oxime (25). Iron will not be extracted if reduced with powdered silver before the ether extlaction (24). Also, gallium can be separated from ferric iron after the ether separation by addition of excess sodium hydroxide to the gallium chloride solution left after evaporation of ether (22). Gallium may be determined fluorimetrically on a chloroform extract of a gallium hydroxyquinolate extracted from a solution buffered to a pH of 2.6 to 3.0 (24). More satisfactory, however, is the spectrophotometric method by which the light absorption of the chloroform extract is measured at 393 to 402 mp (26). Because of the high dollar value of metallic gallium analytical methods for the determination of impurities should be quite sensitive in order to limit the consumption of metal used in analysis. The spectrographic method has been found to be quite satisfactory for this purpose. Approximately 25 mg. of sample is placed in the crater of a graphite electrode and volatilized completely in a d-c. arc. Gallium lines at 2625,2632, or 3004 A. are used as internal standard lines, the choice depending on the relative intensity and distance from impurity lines.
thousand grams of gallium in connection with indium recovery hut no production has been reported since that time. During the thirties Aluminum Company of America inve~tigated processes for removal and recovery of gallium from the sodium aluminate solution in the Bayer alumina plant at East St. Louis, Illinois. However, there was insufficient interest in gallium to warrant continuation . of the study until about 1946, when the investigation was resumed and a practical process developed for producing high-purity metal from the Bayer process solution. Since that time Alcoa has been another source of supply of pure gallium metal. The quoted price for gallium has held fairly steady for the past few years, ranging from about $2.50 per gram (61134per lb.) to $5 per gram, depending on quantity ($6'). At $2.50 per gram gallium is twice as costly as gold. This high cost is due in part t o the small production, but it is probable that gallium will continue to be costly because of the large amount of processing necessary. "Minerals Yearbook" for 1949 lists a t least five suppliers of gallium but also says the demand during 1949 was negligible and only one company reported production. USES
Perhaps the oldest application, since it was suggested by Boisbaudran himself, is the use of gallium as the filler for high-temperature thermometers, because of the extended range of temperature within which it remains liquid. However, thermometers could not be made for high temperatures until quartz capillary tubing became available, which enables the extension of the temperature range from about 700°C. (the limiting temperature for mercury-filled thermometers) to about 1200°C. The first successful high-temperature thermometer using quartz capillary tubing was made by Boyer in the General Electric Laboratories in 1925 PRODUCTION AND PRICES ($7). Gallium-filled thermometers have been ueed The interesting story of the early discovery and pro- to some extent abroad, particularly in Germany ($8). duction of gallium in this country has been told by This is probably the most often mentioned commercial Thompson and Harner (13). I n 1915 Mr. F. G. use. However, it appears doubtful that galliumMcCutcheon observed droplets of metal exuding from filled thermometers will 6nd extensive use, because some of the lead residues of the zinc smelter a t Bartles- temperatures beyond the range of mercury-filledville, Oklahoma. Analysis showed t b e ~ edroplets to thermometers can usually be measured more accurately, be an alloy containing about 94 per cent of gallium and more conveniently, and a t less cost by other means, 6 per cent of indium. He developed a recovery process such as thermocouples. and continued the production of gallium from this Gallium has been suggested as a liquid for manomsource as a hobby. eters which should be more sensitive than mercury During recent years, Eagle-Picher Lead Company manometers by reason of the lower density (6.09 has been a consistent supplier of gallium. versus 13.5). Two difficulties are the tendency to For a number of years prior to World War 11there wet surfaces and the danger of freezing, with consewas some production of gallium in Germany from ger- quent expansion. manite ore and as a by-product from copper refining. A use for gallium, suggested quite early, is in m d During the thirties small amounts were imported into gam-type silver-tim dental alloys to replace mercury, this country for experimental purposes. since it wets the tooth surface and is nonpoisonous. During the period 194345 Anaconda Copper Mining At one time there was some fear of poisoning from deuCompany, Great Falls, Montana, produced several tal restorations using mercury alloys but such danger
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has been disproved ($9). It was hoped that a filling might be obtained that would stay bright and not darken, which would be a great advantage. To date this has not been realized. A gallium-nickel-silicon alloy was reported to show promise for dental fillings (26). Gallium is being used quite successfully in gold sdlovs ~~- eold- - - - ~ " for dental restoration work. These are ---~platinum-palladium-indium-gallium alloys in which the indium-gallium combination is said to reduce the melting point and surface tension, and promotes degassing and soundness of casting with less tendency to oxidize or tarnish. Both strength and ductility are said to be increased without affecting the proportional limit or necessary springiness (30). This application appears promising as a continuing commercial use for gallium. Low-melting alloys of gallium have been recommended for fuses and fusible parts of lire alarm or other systems. Many such systems are, however, satisfied adequately by alloys of cheaper and more readily available metals. The expansion of gallium on freezing has been suggested as a means for obtaining high pressure a t low temperatures (31). Bismuth or water could also serve. This physical characteristic has also suggested the use of gallium in type-metal alloys. In addition, the convenient melting temperature recommends its use in investment casting of small parts. The ability of gallium to wet most surfaces led to the suggestion that it be used for mirrors ( S f ) . A fairly good mirror surface can be produced by simply brushing gallium onto clean glass, particularly if the gallium contains a small amount of zinc. However, more uniform deposition can be attained by evaporating at low pressures, around mm. of mercury (33). In this manner films have been prepared having a light transmission of 4 per cent and a reflectance of about 55 per cent. As might be expected, such a film is soft and has little resistance to abrasion. A peculiarity of these films is that they can be heated for several hours at 300°C. with no tendency to flow, although the temperature is well above the melting point. Such films may be protected by plastic coatings. Very little practical use has been reported for the propensity of gallium t o coat surfaces or to form mirrors, alrhough it is suspected that promising appliwrions may lit, in fields involving surlncc eNects, as for cmmple, lithography. Gallium is said to have found limited use as an excitant in phosphors for fluorescent lighting and luminous paints The metal has been investigated as a heat-exchance medium for high-temperature applications ($6). Its high thermal conductivity, low vapor pressure, and thermal stability recommended it for uses of this nature. During 1947 and 1948 there was much interest in a proposal to use liquid gallium as a coolant, or heat exchange medium for nuclear energy. Howevcr, this interest subsided when it was generally realized that gallium would continue t o be scarce and very costly. ~~
(s).
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There was also the problem of finding suitable metals or alloys to resist the corrosive effects at the operating temperatures. Tantalum resists attack by gallium up to 450°C. and tungsten to 800°C. (6). Gallium does not attack certain ceramic materials, notably quartz, in which it mav be handled to near the melting point of quartz, and aiso in sintered alumina, which it does not tend to wet. Gallium was early proposed as a constituent of vapor lamps because of its rich ultraviolet spectrum (35). Gallium alloys emit electrons a t relatively low temperatures, a property which should be useful. However, little has been published on potential uses in electron tubes. A recent patent (56) involves the use in minor amounts in the selenium rectifier. Gallium is used instead of mercury as a liquid seal in the inlet system of mass spectrometers when analyzing hydrocarbons of high boiling range. The system can be operated at 400°C. with no vaporization of the molten metal (37). There are few uses for gallium salts, although a number of uses have been suggested or investigated on an experimental basis. Gallium chloride is claimed to be a superior catalyst for the polymerization of vinyl ethers (38). Perhaps the most interesting of recent developments is the application involving the use of the radioactive isotope, GaT2,in the diagnosis and treatment of bone cancer (59).
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LITERATURE CITED (1) WEEKS,MARYELVIRA,"Diseovely of the Elements," J. CHEM.EDUC.Press, Eaaton, Pa., 1948. L. S., J. Am. Chem. Soc., 61, 3122 (1939). (2) FOSTER, A. W.. AND H. R. ENGLE,ibid., 61, 1210 (3) LAUBENGAYER, (19391. . (4) FOSTER, L. M., AND H. C. S T ~ P Fibid., , 7 3 , 1 5 W (1951). (5) FRICKE,R., AND W. BLENCKE,Z. anorg. allgem. Chem., 143, 183-200 (1925). (6) "Liquid-Metals Handbook," Atomic Energy Commission, U. S. Government Printing Office, Washinpton. . D. C., June 1, 1950, p. 31. (7) "Metds Handbook," Amencad Society for Metals, Cleveland, Ohio, 1948, pp. 20-1. J. C., U. S. Patent 2,270,193 (1942). (8) MCDONALD, (9) A.I.M.E. Teeh. Publ. No. 1247 (1940). (10) SPICER,W. M., AND H. W. BARTHOLOMAY, J. Am. Chem. Soc., 73, 868-9 (1951). R. W., Nature, 164, 153-4 (1949). (11) POWELL, T ,M., J. Chem. Soc., 1937, 655-673. (12) G O L D S ~ I DV. A. P., AND H. R. HARNER,J. Metals, 191, (13) THOMPSON,
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(15) (16) (17) (18) (19) (20) (21) (22) (23)
STEINBERG: R. A., J . ' A ~ K~eseamh,57, 569-74 (1938). STEINBERG, R. A,, Plant Physiol., 21, 42-8 (1946). BELL, Mamrce, British Patent 622,560 (1949). REYNOLDS, F. M., Chem. Products, 13, 152-3 (1950). HOPFMAN, J. I., J. Hat. Bur. Standards, Research Paper RP-734 (1934). BOROVIK AND SOSEDKO, Compt. rend. mad. an'., U.S.S.R., 14, 31 (1937). REZNYAK, P. A,, AND Z. V. MIRONOVA, Tsvetnye Metal. (Soviet Non-ferrous Metals), 1940, No. 12, pp. 6 0 4 . Swrm, E. A,,J. Am. Chem. Soe., 46, 2375 (1924). KNOWLES, H. B., BUT.Standards J. Research, 9, 1-7 (1932); Chem. Abstmts, 26, 5273 (1932).
APRIL. 1952 E. B., Anal. Chem., 19, 63 (1947). (24) SANDELL, (25) MOELLER,T.,AND A. J. COHEN,ibid., 22, 686 (1950). (26) " ~ i n e r a l sYearbook," U. S. Bureau of Mines, 1949, PP. 1310-11. (27) BOYER,S., Ind. Eng. Chem., 17, 1252 (1925). (28) SMITHELLS, C. J., British Intelligence Objectives SubCommittee Report 345, 1916. , AND W. T. SWEENEY,Dental Cosmos, 73, (29) S O ~ D E RW., 1145 (1931). (30) WILLLMS,R. V., Williams Gold Refining Ca., Buffalo, New York, private communication. , AND L. X*N, J. phv. (U.S.S.R.), 53,193-200; (31) ~ n z m vB., J . Ezptl. Theoret. Phys. (U.S.S.R.), 14, 43947 (1944); Chem. Abstracts, 39, 2915 (1945).
162 (32) EINECKE,E., "Daa Gallium," Leopald Voss, Leipzig, Germany, 1937, p. 127. (33) Bausch & Lomb Optical Co., private eommnnication. (34) DEMENT AND DAKE,"Rare Metals," Chemical Publishing Co., Brooklyn, N. Y., 1946, p. 28. (35) VOGEL,T. W., D. R. P. 217951 (1910). (36) BLACKBURN, W. E., U. S. Patent 2,496,692 (1950). (37) O'NEAL,M. J., AND T. P. WIER,Ana2. Chem., 23, 830-43 (1951). (38) GROSSER, FREDERICK, U. S. Patent 2,457,661 (1948). (39) DUDLEY,H. C., Naval Med. Research Inst., Rept. NM011-013, Sept. 6, 1949.
